Base oil for fluid and grease lubricants is required to perform stably over a prolonged period of time, that is, it is required to have low volatility, excellent heat and oxidation stability, good cold-start properties (low-temperature fluidity), and high viscosity index (wide range). It is no exaggeration to say that an ultimate object of base oil is to have the outstanding properties of low volatility and low viscosity. A large number of compounds have been proposed as lubricant base oil which satisfies these properties. Some of them have succeeded to some extent, but none has attained the ultimate object of satisfactory performance in a range of low viscosity and low volatility at the present time. Acquiring low viscosity and low volatility at the same time implies acquiring two mutually exclusive properties at the same time: for example, an attempt to reduce viscosity of a molecule while maintaining the same structure would reduce the molecular weight, but inevitably increase the volatility.
A polyol ester exists as a product claimed to be free from the aforementioned defect. A polyol ester is bulkier than hydrocarbon-based synthetic oil such as poly-α-olefin and is lower in viscosity than poly-α-olefin when compared at the same viscosity. Moreover, a polyol ester shows excellent heat stability as the raw material alcohol for it such as neopentyl glycol and pentaerythritol does not have a hydrogen atom at the β-position. However, a polyol ester is generally inferior in low-temperature fluidity to and lower in viscosity index than a diester. Hence, a polyol ester cannot satisfy the aforementioned properties.
Patent document 1: JP2003-321691 A
Patent document 2: JP2-041392 A
A polyol ester obtained by the reaction of 4-diethyl-1,5-pentanediol with n-octanoic acid is proposed in patent document 1. This synthesis was carried out to improve the molecular flexibility of the neopentyl group and carbonyl group. However, even the procedure described in the document is unable to yield a polyol ester capable of satisfying the aforementioned properties.
An ester obtained from a dibasic or tribasic acid of 10 or more carbon atoms and a polyalkylene glycol monoalkyl ether is proposed in patent document 2. However, a diester of this kind shows high viscosity as the number of carbon atoms is large. A compound cited in the document has a viscosity of 14 cps or more at 50° C. and it cannot be said to have a characteristic property of low viscosity.